Among various proposed geoengineering methods, ocean alkalinity enhancement (OAE) stands out as a unique solution. By mimicking natural weathering processes, OAE can simultaneously enhance oceanic carbon uptake and mitigate ocean acidification. However, the full efficacy, potential side effects and material selection of OAE restrict large-scale implementation. To evaluate the efficacy of OAE through natural pathways via rivers, we applied a 5-fold alkalinity flux increase (OWE5) at the mouths of global rivers from 2020 to 2100 in a fully coupled Earth System Model under a high-emission scenario (SSP585). In additional sensitivity tests, the flux was increased to 7.5- (OWE75), 10-fold (OWE10), or restored to the control level (OWE0) in 2050. Compared to the control run, global mean surface pH increased by 0.02, 0.03, 0.04, and 0.006; the oceanic inventory of dissolved inorganic carbon (DIC) increased by 5.39, 7.41, 9.50, and 2.06 Pmol; and atmospheric CO₂ concentration decreased by 29, 40, 51, and 11 ppmv under OWE5, OWE75, OWE10, and OWE0, respectively, by the end of the century. The most significant responses to OAE were observed in coastal regions, as well as in the Indian and North Atlantic Oceans. Our simulations demonstrate that OAE via rivers is an effective and practical method. We simultaneously simulated the performance and benefits of one of OAE materials, magnesium carbonate (MgCO₃) foam, and verified its efficiency and safety before the large-scale implementation of OAE for carbon sequestration.